Art of electrolytically treating metal to clean, level, smooth, polish and/or protect the surfaces thereof



,Jan- 13, 1959 J.YJ. BAIER Erm. 2,868,705 ART OF ELEOTROLYTICALLY TREATING METAL To CLEAN, LEVEL, SMOOTH, POLISH AND/OR PROTECT THE suRFAcEs THEREOF Filed Jan. 19, 1955 2 Sheets-Sheet l raf/fade? 242/? Jan.l 13, 1959 J. J. BAIER ET A1. 2,868,705

ART OE ELEOTROLYTIOALLY TREATTNG METAL TO CLEAN. LEVEL, SMOOTH, POLISH AND/OR PROTECT THE 4'SURFACEIS THEREOF Filed Jan. 19. 1955 2 Sheets-Sneet 2 Vm Uil!! aes, i'

ART F ELECTROLYTICALLY TREATING METAL TO CLEAN, LEVEL, SMOOTH, POLISH AND/ 0R PROTECT THE SURFACES THEREOF Y John J. Baier, Manhasset, and ElrneiriWodetzky,

` Freeport, N. Y.

The present invention relates to theart of electrolytically treating metal to clean, level, smootlnpolish and/or protect the surface thereof.

It is believed that inthe anodic treatment of metals, a viscous layeror film of high electrical resistivity is formed on the surface of the anode being treated during the passage of current through the electrolyte. Since the electrolyte is of comparatively low electrical resistance, the formation of a layer of comparatively high resistance on the `anode surface, causes the anodic potential in the different regio-ns of the surface being treated to vary according to the extent to which these regions project into said layer. This causes the salient points on the surface to fuse at a rate according to their depth, thereby levelling olf said points until an equipotential condition is atttained over the surface. At this stage, the surface will be levelled and smoothed ol.

One object of the present invention is to provide new and improved electrolyte for anodically treating a metal surface, which is fast acting and lends itself to continuous and mass production, which renders the surface even and extremely smooth, which imparts a permanent high lustre or polish having high reflectivity to the surface, which so acts on the surface as to impart thereto high resistance to oxidation and corrosion and high weldability, and which can be used for long periods yunder stable electrolytic conditions and without regeneration of the electrolyte.

Various other objects of the invention are apparent from the following description and from the accompany ing drawings, in which Fig. 1 is a diagrammatic View of an electrolytic tank, and associated parts for carrying out a process in its primitive form according to the present invention;

Fig. 2 is a graph showing Ithe current density-voltage relationship of the different stages or phases of the process illustrated in Fig. 1;

Fig. 3 is a diagrammatic view of a cathode and anode assembly and design showing one way in which the electrolyte of the present invention can be used in cleaning and polishing at continuously moving strip metal of substantial width;

Fig. 4 is a diagrammatic view of a cathode and anode assembly and design showing another way in which the electrolyte of the present invention can be used in cleaning and polishing flat metal plate of comparatively narrow width;

Fig. 5 shows diagrammatically the outlines of an airplane and especially vthe fuselage and part of the Wing which may be cleaned and polished lby the electrolyte of the present invention;

Fig. 6 is a side elevation of one of the airplane Wings shown in Fig. 5 and shows diagrammatically the surface of said wing being cleaned and polished by an electrolytic box assembly containing the electrolyte ofthe present invention, said electrolytic box assembly beingshown in section; l

Fig. 7 is a section of a cathode and anode arrangement anddesign by which the burrs on a workpiece may be assauts Patented Jan. 1,3, 1959 easily removed by the electrolyte of the present invention;

Fig. 8 is a section of a cathode and anode arrangement and design by which the internal surface of a tubular member, such as that of the cylinder or chamber of a combustion engine, steam engine or pump, may be cleaned and smoothed by the electrolyte of the present invention; and

Fig. 9 is a section of a cathode and anode arrangement and design by which a surface of intricate or complicated contour may be cleaned, smoothed and polished by a series of cathode elements shielding the different sections -of the contour and supplying variable voltage according to the nature of the contour sections, using the electrolyte of the present invention.

In using the electrolyte of the present invention in the manner illustrated in Fig. 1, a tank 10 is provided containing an electrolyte 11, the special nature and composition of which will be described hereinafter. The metal piece 12 with its rough surface 13 to be treated, is suspended in the electrolyte as the anode. The irregularities on the surface 13 of this metal piece 12 are greater than would be encountered in practice, and are shown herein in exaggerated form only for the purpose of clarity in illustration.

The cathodes 14 which flank the metal piece 12 may be of any well known inert material, such as stainless steel. When direct current is applied between the cathodes 14 and the anode 12, the irregular surface 13 will be evened off and smoothed. The mechanism for this operation is believed as follows:

With the application of direct current, a viscous layer 1S of comparatively high electrical resistance is formed over the rough surface 13 of the metal plate 12, consisting of a mixture of ions from the electrolyte and ions from the dissolved anode. This layer 15 has its surface 16 opposite the rough surface'l3 substantially flat, so that the vthickness of this layer from the salient points on this surface to the layer surface 16 varies according to the depth of vthese salient points. Since the electrical resistance of the electrolyte is very low, the electrical resistance of the layer opposite these points varies according to the depth of these points. The salient points of greater depth on the surface 13 of the metal piece 12 therefore receive current of greater density than the points of lesser depth. The net result is that the high points on the rough surface 13 of the metal piece 12 are dissolved at a faster rate than the low points, thereby gradually levelling oi this surface until a conditionof potential equilibrium is attained. When this stage of equilibrium is reached, the surface will be ilat and smooth, as shown in dot and dash lines in Fig. l, and will have all the other desirable characteristics to be described.

An important feature `of the present invention is the composition of the electrolyte 11 employed in the process described. The basic ingredients of the electrolyte are phosphoric acid and chromic acid. If the phosphoric acid is used alone without the chromic acid in the electrolyte, the layer 15 of high viscosity developed will contain numerous gas bubbles which cause spotty polish ing. However, the viscousvlayer 15 produced by the phosphoric acid alone has vsome goodA characteristics which it is desirable to retain. For example, it has a low degree of adherence, and therefore is constantly renewed by successive new layer formations. lf bubbling in this layer can be kept down, a smoother surface will be produced,'but this requires a comparatively long time to attain. l

If the chromic acid or its anhydride is used in the electrolyte ,without the phosphoric acid, the resulting layerf15 will be free of gas bubbles. However, if'so exclusively used, the acid has a tendency Vto produce wavy surfaces which nevertheless have good reectivity.

It has been found in accordance with the present invention, that by combining the phosphoric acid and the chromic acid in the electrolyte, the desirable characteristics of both are attained and the undesirable characteristics ofeach are substantially obviated or materially reduced. At the same time, it has been found-that the actionvof etectively cleaning, polishing or otherwise treating the surface of the metal piece is faster with the combination ofchromic acid and phosphoric acid, than it is with either one of these ingredients alone.

The proportions of chromic acid and phosphoric acid of the electrolyte depend on the type of metal to be treated and the finish that is desired. The phosphoric acid should preferably beat least 60% by weight'(on a purebasis) of the electrolyte and the chromic acid should beat least 1.7% (on an anhydride basis). It has Vbeen found, that a good working range for these ingredients is 76.4 to 94.3 parts'of phosphoric acid ,on the basis of an 84 to 85% syrup `to 1.9-13.2 parts of chromic acid on the basis of the anhydride.

In addition to the specific ingredients described, it hasbeen found in accordance with the present invention, that the addition of a small amount of sulfuric acid and a still smaller amount of hydrofluoric acid in the electrolyte, increases the current carrying capacity of the electrolyte, provides better current distribution, and` prevents certain granulation on the surface of the anode. Also, with deterioration of the electrolyte due to age, the resistance or strength of the anode layer is increased, and this has an adverse effect on the process. This sul furic acid and hydrouoric acid act as stabilizers to retard materially this deterioration. A good working range for the sulfuric acid is 0.7% to 2.7% of the final electrolyte and for the hydrofluoric acid is 0.004% to 1.014%.

As the electrolyte ages, the voltage necessary to attain the necessary polish increases, due to the increase in the resistance of the electrolyte. This is believed due to the deterioration of the chromic acid resulting from its reduction. It has been found in accordance with the present invention, that the addition of a small amount of sodium dichromate to the electrolyte, retards materially the reduction of the chromic acid and prolongs the useful life of the electrolyte to a substantial extent, so that satisfactory cleaning, smoothing and polishing is attained l.

over the life of the electrolyte, Without material increase in voltage. A good working range for the sodium dichromate is 0.6 to 2.6% of the electrolyte.

If under high current densities, excessive gassing results due to the resistance of the electrolyte being too low,

an ounce or tWo of acetic acid may be added to the electrolyte as a stabilizer. This will tend to increase the resistance of the electrolyte and to decrease the cur- 4rent density, so that gassing will cease.

The electrolyte described should be maintained at a i temperature of between 175 F. and 185 F. Below this temperature range, the process will be too slow and above this range, the electrolyte in the vicinity of the viscous layer 15 will bubble excessively. To maintain the electrolyte 3.1i at the required temperature, cooling of the electrolyte is resorted to, as for example, by cooling the tank 10 with cooling water.

It is also desirable that the electrolytic bath be kept underconstant circulation while in use.

In order to assure an electrolyte mixture, which is chemically andphysically stable, the different ingredients of the electrolyte must be put together in a definite manner. To that end, the bulk of the phosphoric acid and ofthe chromic acid preferably in anhydride form are mixed together to produce mixture No. 1. This mixture No. l is produced in a specic instance by mixing the phosphoric acid in the form of an extra pure syrup 84-85%, having a specific gravity of 1.70 equivalent to 59.5` B. with acid chromic anhydride. The proportions by weight of the ingredients in mixture No. 1 on the basis of the final electrolyte are preferably as follows:

MIXTURE NO. 1

Poshphoric acid (84-85%)-73.5% to 91.4% of the final electrolyte Acid chromic anhydride-1.7% to 11.7% of the final electrolyte A MixtureNo. 2 is produced comprising phosphoric l, acid, acid chromic anhydride, sulfuric acid, hydrofluoric acid, dichromate of sodium, and Water. In this Mixture No. 2, the proportions by weight of the ingredients are preferably as follows, on the basis of the linal electrolyte:

MIXTURE NO 2 Percent Phosphoric acid 2.9-6.9 Acid chromic anhydride -e l 0.2-1.5 Sulfuric acid a 0.7-2.7

Hydrouoric acid 0004-0014 Dichromate of sodium 0.6-2.6 Water (suicient to make up 100% in the final electrolyte) 0.1-2.1

After Mixtures No. 1 and No. 2 have been separately formed as described, they are combined.

A speciiic example of an electrolyte formed in accordance with the present invention is as follows:

MIXTURE NO 1 1 gallon of `phosphoric acid Extra Pure Syrup 84-85% (1.70'sp. gr.=59.5 B.) equivalent to 83.5% of the.

final electrolyte 500 grams of acid chromic anhydride equivalent to 6.7%

of the final electrolyte MXTURE NO. 2

200 cubic centimeters of phosphoric acid Extra Pure Syrup 84-85% (1.70 sp. gr.: 59.5 B.) equivalent to 4.9% of the final electrolyte 40 grams acid chromic anhydride equivalent to .5% of final electrolyte cubic centimeters sulfuric acid equivalent to` l.

final electrolyte 1 cubic centimeter of hydrofluoric acid equivalent to 0.009% of final electrolyte 120 grams of dichromate of sodium equivalent to 1.6%

of final electrolyte cubic centimeters of Water equivalent to 1.1% of the linal electrolyte In forming the nal electrolyte, cubic centimeters of Mixture No. 2 are combined with eaclrgallon of Mixture No. 1. v

In the operation of carrying out the surface treating process indicated above withthe electrolyte described in the specific example, while the electrolyte is maintained at F. and agitated, direct current is passed therethrough between the anode and cathode immersed theren1.

it has been found that with an electrolyte of the type indicated above, the average metal dissolution on normal metals, such as aluminum and steel, is in the neighborhood of 0.0003 per minute of current application with a current density of 0.2 to 0.5 ampere per square inch. It is seen that the entire process can be accomplished in "f a matter of seconds, in ,certain cases as little as 3 seconds, on work, where the roughness is very minute. Even in extreme cases of roughness, the entire process need not take over 30 seconds to accomplish-a smooth highly polished surface. A surface appears polished to the eye, when the maximum depth of the pores or rough ness is smaller than one-half of a light wave of visible light (one light wave is equivalent to 5000 Angstrom units).

In carrying out the electrolytic pro-cess of the present invention with ythe electrolyte of the present invention. the

viscous layer is brought up to its maximum temperapractical limits of voltage and current density, byincreasing the voltage at a rate, rapid enoughl to maintain the temperature of said layer a few degrees (5 to `7 F.) above that of the metal anode. By carrying out this initial phase of the cleaning and polishing/cycle as'described, a fastercleaning and polishing results, thereby cutting down the timing of a vcomplete cleaning and ,polishing cycle to a matter of seconds.

In Fig. 2 is shown a current density-.voltage curve which is characteristic of the process. During the entire cycle, since the temperature of the electrolyteas well as of the viscous layer 15 is increasing, the resistance of said viscous layer is also increasing. Therefore, the voltageto attain the necessary current density at the anode 12 must also be increased. During the initial phase indicated by the part a of the curve, the voltage is increased at a rapid rate to increase the temperature of the viscous The voltage is increased progressively from 0-18 volts during 3 seconds. The current density goes up to a maximum of 0.3k ampere per square inch and then drops to 0.21 ampere.

Phase b The voltage is increased progressively from 18 t-o 28 volts during a period of 3-5 seconds. The current 'density during this phase remains co-nstant at 0.21 ampere per square inch.

Phase c The voltage is increased progressively from 38 to 40 volts during a period of 3`5 seconds. v The current density increases progressively from 0.21 ampere per square inch Phase d The voltage is progressively increased from 40 to 45 volts during a period of 2-3`seconds. The current deni sity increases progressively from 0.50 ampere to 0.75

of the viscous layer. 15 is still comparatively low, the

mains at a steady value for a period indicated by the interval b on the graph of Fig. 2. During this phase b, normal smoothing and polishing action takes place, until a point is reached at which the current ,density starts t0` increase, although the voltage rate increase mightremainthe same. The interval c of this part of the cycle is a transition period during which sporadic polishing takes place and continues into phase d during which'the polishing becomes more efficient and more uniform. Phase d continues with progressive increase in voltageand progressive increase in current density until the current density reaches a .peak and then drops suddenly to a level of current density substantially lthe same as that attained during phase b, to initiate phase e, even'though the rate of voltage increase should remain the same. i During phase e extremely high polish to a' mirror finish is attained and this phase may be continued depending uponl the degree of polishing required. t

yDuring the entire cycle, the rate of voltage increase may be maintained substantially constant, and due to the peculiar characteristics of theelectrolyte, the process will undergo the different phases described, or the voltage rate result.

For normal smoothing and cleaning, the cycle need not be extended beyond phaseA b. However, for extreme cleaning and extremely high polishing, the cyclevcan be extended into phase e. y

An example of a cycle which may be employed with the Velectrolyte described in the specic example above, and inconnection with an aluminum workpiece to be cleaned and polished, ,islas follows:

In any case,- the temperature of the viscous layerv In addition, undesirable bubbling of theV electrolyte in the vicinity vof the viscous layer 15 might ampere. At the end of the third second of this phase d, the current density will drop suddenly back to 0.21 ampere per square inch.

Phase e the processing tank k10, and as thefelectrolyte is depleted by adherence of some electrolyte tothe treated metal piece withdrawn from the tank, by running into the tank by gravity or pressure, suicient electrolyte to make up the deficiency. VIf no electrolyte is removed from the treating tank, its strength `may be kept'up by the additiony of chro-mic acid'and sodium dichromate.

After one month of normal continuous use, it is advisable to replace the entire electrolyte. The condition of the electrolyte necessary to replace it can be ascertained -by noting the color ofthe electrolytepwhich changes from Ibrown to green and which also manifests increased foaming, when it has deteriorated to the stage where replacement is required. The stale electrolyte can be used as a precleaner for the metal workpiece to be cleaned before it is immersed in the fresh electrolyte, by heating the stale electrolyte to a temperature of between 200- 210 F.,-without using electric current.

The action of the improved electrolyte in the process described is fast acting and can be'extended to produce a surface which is'notonly smooth but has a high lustre or polish. Tests have shown that the surface produced by the process of the present invention has characteristics not obtained by smoothing and polishing to a comparable degree -by jmechanical means.

retain its lustre and high retlectivity almostindeiinitely, while a mechanically finished surface of equal initial smoothness and polish will' lose its polish in a short time. Moreover, a surface treated by the electrolyte of the present invention will be highly resistant to atmos,

pheric corrosion, and stainless steel surfaces so treated, manifest high resistance against corrosion and especially against the action o-f chlorides and organic acids indefinitely, while a mechanically nished surface of equal initial smoothness` and polish will erode during the same period. This improved result is due to the fact that in conjunction with an extremely smooth and highly polished surface, there is produced by the electrolyte of the present invention, an extremely thin transparent pro- For example, a surface f polished by the electrolyte of the present invention will' tective sealing coating on zthe` smooth polished surface. This coating is 'believed to be an oxide, but unlike metals with the usual oxide coatings, the metals treated. as described manifest improved weldability.

It has also been found that a plating appliedin the usual manner to a metal surface cleaned and polished electrolytically in accordance with the present invention manifests better adherence to the base metal, greater durability and resistance to corrosion, and less porosity than in a case of a plating applied in the same manner to a surface, which has been mechanically cleaned and polished.

Theelectrolyte of the present invention can be used forcleaning, sealing and polishing all metals including alloys, except chromium and zinc. By cleaning and sealing is meant cleaning and smoothing to the extent of preventing further oxidation of the metal under normal conditions of usage. The process can be used, for example, to clean and .polish aluminum, steel, brass `and copper, and in addition it will seal the surface of the metal against deterioration in normal atmospheres.

In treating an anodically connected work sheet, it has been found that a flat cathode sheet on either or both sides of the work sheet does not produce as even a current distributio-n as might be desired.V For example, it has been found that at the sides of the anodically connected sheet, the current density isgreater than in the intermediate section-of the sheet. It has been determined that a contoured cathode sheet adjacent to theflat anode sheet to be electrolytically treated, produces a more even current distribution. be contoured by corrugations in the sheets, especially in the case of a comparatively' wide anode sheet, or by arching the'cathode sheets, so that the sides of said sheets slant away from the sides of the anode'sheet, especially in the case ofa comparatively narrow anode sheet. Fig.

3 shows one of these arrangements in connection with- For the arrangement of Fig. 3, the cathode sheets 18` are corrugated in a direction transverse to the direction of motion of the anode sheet i7. The corrugations of the cathode sheets 18 are preferably of sinusoidal form, and in a specific example the depth d of each corrugation is between-3% and 1/2; the length e from one point on a corrugation unit to the corresponding point on the next corrugation unit is between 7s" and 1% and the nearest distance f of the corrugated'sheets 1S from the anodel 17 is between l and 21/2. En a specific installatio-n, the distances d, e and f, may be 3%", 7a and 1%" respectively.

Where both faces of the anode sheets 17 are to be finished, two corrugated cathode sheets 18 are required on opposite sides of said anode sheet as shown.

Fig. 4 shows an arrangement in which the cathode sheets are contoured simply in connection with a comparatively narrow anode `sheet up to a foot wide, to attain luniform current density across the anode. In

this arrangement, there is provided a flat anode sheetl 20v to be cleaned and polished on opposite faces. This sheet 20 may be ystationary while being electrolytically treated or may be a continuous strip being advanced lengthwise while being treated. On opposite sides of this ano-de sheet 2t) and equally spaced therefrom are two cathodensheets 21, arched and symmetrically arranged-with their-convex faces turned 'towards the anode sheet,.so that the sides of said cathode sheets slant outwardly away from the sides of the anode sheet. This bringsthe middle section'of the cathode sheets 21 closer The cathode sheet or sheets may,

`"said boxto be pushed lalong said surface.

tofthe'-anodefsheetfiathan `the side sections, the curvatureoffsaidicathode sheets being such as to attain uniform.. distribution.,of.current density acrossthe sheet. The"cathodetsheets.5211.may, for instance, be circularly curved, and in a specific example,k with an anode sheet 20,16 inches ;wide, :the intermediate sections of the cath` ode sheets 21,-may be spaced 2 inches from the confronting faces ofthe anode sheet 2i)` and the sides of the cathode sheetsrat the Vregions directly opposite the side edges of the anode sheet may be spaced 31A. inches from the confronting` faces of the `anode sheet.

The electrode arrangement of Fig. 4 when immersed in anV electrolyte 22 preferably of the improved character described,v and when the electrolytic circuit is closed, delivers to the anode sheet 2@ current of uniform densityacross the-width of said anode sheet.

The` electrolyte of the present invention is useful, for example, in connection with the cleaning. polishing and sealingfofl the external surfaces of airplane bodies and ,wings made of aluminum, as shown in Figs. 5 and 6. Forrthatpurpose, there is shown an airplane 23. with a fuselage 24 and wings 35. The top section V26 of thel fuselage V24 is usually painted to preventabsorption of heat, so that this section need not be cleaned orpolished electrolytically or-by any-other process. The surfaces 27 of the wings 25, as well as the sides 2S and bottom 29 of the fuselage 24, require cleaning, polishing andv sealing.

For cleaning the wing surfaces 27, there is provided a portable box-30, shown of rectangular form and inverted over the surface to 'be treated, this box having a gasket 31vof suitable resilient material, such as neoprene, around its bottom edge for hermetically sealing the box tothe surface to be treated,` while permitting This box 30 contains-anelectrolyte 33,'which is preferably of the improved composition described above, and a corrugated sh'eet 34 `ofthe type shown in Fig. 3 or of the arched type shown in Fig. 4, cathodically connected to a source 35 of direct current, while 'the wing surface 27 to be treatedV is anodically connected to said source.

For cleaning, polishing and sealing the surface .27, the box 30 is moved over this surface at a rate to effect the necessary treating operation. Because of the quick acting nature of the electrolyte 33, requiring only a matter of afew seconds to effectively treat any particular area, the cleaning, polishing and sealing operation o-f the. entire surface can be accomplished in a comparatively short time.

The surfaces 27 of the airplane wings 25 are generally iiat, so that a box 30 'presenting a substantially flat open end or rim with the sealing band 31 around it will suffice to maintain the desired sealing contact with said surfaces, as the box is pushed along said surfaces. There are, however, certain parts of the lexternal airplane surfaces that have curvatures which cannot sealably receive a box with a flat open end, as described, but which require a box having an open end of corresponding curvature to t somewhat conformably and sealably over these curved surface parts. For example, the concave junctures- 36 between the wings ZS'and the sides of the fuselage 24 are of this curved nature. Also, certain parts of the fuselage 24 itself, as for example, the bottom part 29, may be curved enough to require an electrolytic box with a curved rirntto conform sealably therewith. In any case, there are no more than two or three general types of contours requiring electrolytic boxes with rims of corresponding contours, so that the entire cleaning, polishing and sealing operation can be carried out with only two or-three types of electrolytic boxes.

Theelectrolyte ofthe presentvinvention lends itself to selective Vaction on restricted sections of a surface of a metal piece, especially where such a piece has a shape more intricate than that of a plain sheet. For example,

after certain shearing or machining operations, a metal piece may have burrs along the sheared machine edges, which require to be removed. To remove such burrs, there is contemplated the use of a cathode, shaped to concentrate its action on these selected burred regions of the metal piece, so that other parts of this piece are not substantially affected by electrolytic action. Fig. 7 shows the use of the electrolyte for that purpose. In the arrangement of Fig. 7, the metal piece 40 is shown consistingof a sleeve or collar, having a cylindrical section 41 and a ange 42. This metal piece 40, after machining, will have peripheral bnrrs around the edges 44, 45 and 46. To electrolytically remove these burred edges 44, 45 and 46, there is provided an anode rod 47 on which the metal piece 40 is impaled, so that this metal piece is anodically connected in the electrolytic circuit. Around this metal piece 40 and concentric therewith, is an annular cathode 48 having peripheral annular sections 49, 50 and 51 projecting radially in wardly in position directly opposite the burred edges 44, 45 and 46 respectively, so that the electrode action of the cathode 48 is concentrated on these edges. The arrangement described is immersed in an electrolyte 52 preferably of the improved character described.

The general arrangement shown in Fig. 7 lends itself to deburring action on a mass production basis. For example, the cathode 48 may be stationary but the metal pieces 40 to be deburred may be slipped over the anode rod 47 in numbers and moved step by step thereon, to move said metal pieces successively in position inside the cathode 48, to be deburred thereby, and then successively out of said cathode when each deburring operation is completed.

The electrolyte of the present invention has great utility in connection with the cleaning and smoothing of the internal surfaces of tubular members, as for example, the inside bore surface of the cylinders of combustion engines or pumps. Fig. 8` shows such an arrangement, comprising an engine cylinder 55 shown somewhat diagrammatically, having a cylindrical bore 56 with a cylindrical internal surface 57. The bottom of this cylinder 55 is closed by a seal 58 and a head plate 60 is placed over the cylinder. This cylinder 55, which is anodically connected to a source 61 of direct current, is filled with electrolyte 62, preferably of the type described above, and immersed in the electrolyte in this cylinder is a cathode 63 of cylindrical form concentrically placed in relation to the internal surface 57. By subjecting the electrolyte to voltage in the manner described above, the internal 'surface57 of the cylinder engine is cleaned and smoothed to a much greater degree than is possible by machining. i

In cases where the contour of the anodically connected workpiece to be cleaned and polished is intricate, it is difficult to clean and polish electrolytically uniformly over the contour of said workpiece. Uniform cleaning and polishing for such a complicated contour may be accomplished by breaking up the cathode into a series of separate elements in conforming shielding relationship with respective sections of the workpiece to be cleaned and polished and by applying separate voltages to these cathode elements, differing in values according to the configuration of the sections shielded. Fig. 9 shows an embodiment of this feature, comprising a workpiece 69 connected as an anode to a source 70 of direct current and having an intricate surface 71 consisting of a series of sections 72 of different contours, some of which may be in the form of rounded projections or beads and some of which are flat. Placed opposite each of these contour sections 72 and shaped therefrom in shielding relationship thereto, are a series of cathode elements 73 conforming in shape with the respective contour sections. These cathode elements 72 are separately connected to the source 70 of direct current through respective rheostats 75, by which the voltages on the individual cathode elements, and in turn the required current densities may be selected according to the shape of the cathode elements, to attain uniform cleaning and polishing over the entire contoured surface of the workpiece 69. The arrangement shown in Fig. 9 is operated in an electrolyte 76 preferably of the improved type described above.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed is:

1. An electrolyte for use in anodically treating metal surfaces, consisting essentially of phosphoric acid and chromic acid and containing 0.7 to 2.7% by weight of sulfuric acid and 0.004 to 0.014% of hydrofluoric acid.

2. An electrolyte for use in anodically treating metal surfaces, comprising at least 60% by weight of phosphoric acid on a pure basis, 2 to 12% of chromic acid on the basis of the anhydride, 0.7 to 2.7% of sulfuric acid and 0.004 to 0.014% of hydrouoric acid.

3. An electrolyte for use in anodically treating metal surfaces comprising at least 60% by weight of phosphoric acid on the pure basis, 2 to 12% of chromic acid, 0.7 to 2.7% of sulfuric acid, 0.004 to 0.014% of hydrouoric acid, 0.6 to 2.6% of sodium dichromate and 0.1 to 2.1% of water.

4. An electrolyte for use in anodically treating metal surfaces, consisting essentially of at least 60% by weight of phosphoric acid on the pure basis and 2 to 12% of chromic acid and containing 0.6 to 2.6% of sodium dichromate.

References Cited in the le of this patent UNITED STATES PATENTS 982,037 Crocker Jan. 17, 1911 2,040,618 Mason May 12, 1936 2,549,946 Truehaft et al Apr, 24, 1951 2,550,544 Faust Apr. 24, 1951 2,708,655 Turner May 17, 1955 .FoREIGN PATENTS 530,041 Great Britain Dec. 4, 1940 OTHER REFERENCES The Iron Age, December 21, 1939, pp.` 30-32, and 66, article by Kiefer. 

1. AN ELECTROLYTE FOR USE IN ANODICALLY TREATING METAL SURFACES, CONSISTING ESSENTIALLY OF PHOSPHORIC ACID AND CHROMIC ACID AND CONTAINING 0.7 TO 2.7% BY WEIGHT OF SULFURIC ACID AND 0.004 TO 0.014% OF HYDROFLUORIC ACID. 